Modern hydrocracking catalysts are generally based on zeolitic materials due to their advantages over earlier refractory oxide based materials such as silica-alumina, magnesia, and alumina. Amorphous catalysts have relatively poor activity but higher selectivity for production of distillate range product while zeolite catalysts provide higher activity but poorer selectivity for distillate, particularly for the heavy-distillate fraction. Among other things, the present invention provides a hydrocracking process with superior overall catalytic performance over amorphous silica-alumina cogel catalyst while maintaining the excellent heavy-distillate selectivity and unconverted oil quality of the amorphous cogel catalyst.
In state-of-the-art cogel hydroprocessing catalysts, various combination of metals, their oxides and sulfides, from Group VI-B and Group VIII of the Periodic Table, were precipitated or cogelled with the base. Therefore, the deposition of metals by cogellation was considered superior to impregnation of the metals on the base (post-metal impregnation), since the latter method tended to produce non-uniform deposits of active metals. The increased costs of production inherent in cogellation or precipitation were simply borne in order to obtain more uniform catalyst which had better distillate selectivity and produced a superior quality product.
The present invention provides, among other things, a novel post-metal-impregnated, hydrocracking catalyst made with homogeneous silica-alumina and highly dealuminated ultrastable Y zeolite (USY) which produces a number of unexpected benefits including: the catalyst activity is improved significantly over the state-of-the-art amorphous cogel catalyst, and at the same time the total distillate yield increased. The catalysts of the invention are very stable and showed significantly lower fouling rates than the cogel catalyst.
It is well known that addition of USY to a cogel hydrocracking catalyst generally lowers the distillate yield, particularly the heavy-distillate fraction with 550°-700° F. boiling point range. The catalyst of the present invention is particularly effective with respect to maintaining the yield of a heavy-distillate fraction. The hydrocracked heavy-distillate fraction from vacuum gas oil typically exhibits very high Cetane Numbers of 60-80, while a light-distillate fraction with 250°-550° F. boiling point range exhibits Cetane Numbers of 40-55. In order to achieve a high Cetane Number for the combined diesel fuel, it is desirable to increase the heavy-distillate content in the combined distillate pool. Moreover, the unconverted 700° F.+ fraction in a process according to the invention exhibits high viscosity index indicating that a high quality lubricating oil bas stock can be produced from that fraction.
An early synthetic zeolite Y was described in U.S. Pat. No. 3,130,007 issued Apr. 21, 1964, which is incorporated herein by reference. A number of modifications have been reported for this material, one of which is ultrastable Y zeolite as described in U.S. Pat. No. 3,536,605 issued Oct. 27, 1970, which is incorporated herein by reference. Zeolite Y has been constantly improved by techniques like ammonium ion exchange and acid extraction, and various forms of calcination in order to improve the performance of the hydrocracking catalysts.
To further enhance the utility of synthetic Y zeolite and depending upon the hydroprocessing problem sought to be solved, additional components have been added by means known in the art. U.S. Pat. No. 3,835,027 to Ward et al., which is incorporated herein by reference, describes a catalyst containing at least one amorphous refractory oxide, a crystalline zeolitic aluminosilicate and a hydrogenation component selected from the Group VI and VIII metals and their sulfides and oxides. Ward et al. teach that the added materials enhance the catalytic and denitrogenation activity of the catalyst.
U.S. Pat. No. 3,897,327 to Ward, which is incorporated herein by reference, describes a hydrocracking process using a sodium Y zeolite wherein the Y zeolite has a preliminary ammonium ion exchange to replace most of the sodium ion with ammonium ions. This product is then calcined in the presence of at least 0.2 psi of water vapor for a sufficient time to reduce the cell size to a range between 24.40-24.64 Å. The patent teaches that the catalyst has increased hydrothermal stability by maintaining crystallinity and surface area after calcination, exposure to water vapor or water vapor at high temperatures.
In addition to various catalyst compositions, preparation techniques have been discovered to also affect catalytic selectivity. U.S. Pat. No. 3,867,277 to Ward, which is incorporated herein by reference, discloses the use of a Y type zeolite catalyst in a low pressure hydrocracking process. The catalyst described in the patent requires the Y zeolite to be double-exchanged and double-calcined wherein the first calcination step uses a relatively high temperature (950°-1800° F.) and the second calcination step uses relatively low temperatures (750°-1300° F.) to yield a catalyst that is stable in ammonia environments.
U.S. Pat. No. 3,853,747 to Young, which is incorporated herein by reference, teaches that hydrocracking activity of the catalyst is greater when the hydrogenating component is incorporated in the zeolite in such a manner as to avoid impregnation into the inner adsorption area of the zeolite crystallites or particles. For example, the mixing may consist of stirring, mulling, grinding, or any conventional procedure for obtaining an intimate mixture of solid material. The dispersion of the Group VIB metal hydrogenation component is achieved by adding it to the zeolite in a finely divided but essentially undissolved form. The patent teaches that in some cases the soluble molybdenum or tungsten compounds added to the zeolite by impregnation tends to destroy the zeolite crystal structure and acidity during the subsequent drying and calcination steps. Young teaches, however, that the particle size should range from 0.5 microns to 5 microns.
U.S. Pat. No. 4,857,171 to Hoek et al., which is incorporated herein by reference, teaches a process for converting hydrocarbon oils comprising contacting the oil with a catalyst consisting essentially of a Y zeolite having a unit cell size less than 24.40 Å, a silica based amorphous cracking component, a binder and at least one hydrogenation component selected from the group consisting of a Group VI metal, and/or a Group VIII metal and mixtures thereof.
U.S. Pat. No. 4,419,271 to Ward, which is incorporated herein by reference, discloses a composition matter useful as a catalyst base for supporting active hydrogenation metal components or for catalyzing acid catalyzed hydrocarbon conversion reactions comprising in intimate heterogeneous mixture (1) a modified hydrogen crystalline aluminosilicate Y zeolite having activity for catalytically cracking hydrocarbons and having a unit cell size between 24.25 and 24.35 Å and a water absorption capacity, at 4.6 mm water vapor partial pressure and 25° C., less than 8% by weight of zeolite and (2) a dispersion of silica-alumina in a gamma alumina matrix.
U.S. Pat. No. 4,820,402 to Partridge et al., which is incorporated herein by reference, discloses the use of a highly siliceous large pore zeolite as the acidic component of a catalyst in a process for improved distillate selectivity.
U.S. Pat. No. 5,171,422 to Kirker et al., which is incorporated herein by reference, discloses a process for producing a high quality lube base stock with a USY catalyst with greater than 50:1 silica:alumina ratio.
WO 00/40675, which is incorporated herein by reference, discloses a low pressure hydrocracking process using a catalyst comprising zeolite USY with a framework silica to alumina molar ratio of at least 200:1 preferably greater than 2000:1, and a hydrogenation component.
GB-A-2,014,970 discloses an ultra hydrophobic zeolite Y which has been given a unit cell size dimension of 24.20-24.45 Å by two ammonium exchange steps with an intermediate calcinations step at 550°-800° C. in steam. EP-B-0,028,938 discloses the use of such a modified zeolite for selective conversion of hydrocarbons boiling above 371° C. into midbarrel fuel products having a distillation range of 149°-371° C. Improved manufacturing process for producing 24.25-24.35 Å unit cell size Zeolite Y was disclosed in U.S. Pat. No. 5,059,567 to Linsten et al.
Silica-alumina compounds are well known as catalysts used in hydrocarbon conversion processes. Silica-alumina catalysts such as in the present invention can be used “as is”, particularly in reactions that require acidic catalysts, or can optionally be combined with zeolites, clays or other binders, and inorganic oxides for the cracking of liquid hydrocarbons in cracking reactors such as fluid catalytic crackers and hydrocrackers. Silica-alumina composites have been used commercially for a variety of hydrocarbon processing applications, such as cracking, desulphurization, demetalation, and denitrification.
Amorphous silica-alumina has been prepared previously by a modified cogel process wherein no hydrogenation metals were added during the gellation step. Spray dried, amorphous silica-alumina catalysts were made by the method as described in U.S. Pat. No. 4,988,659, Pecoraro to produce catalysts used in hydrocarbon conversion processes.
The method of preparation of silica-alumina catalysts affects the chemical and physical properties of the catalysts such as activity (such as cracking or isomerization activity), and physical properties (such as pore structure and volume, surface area, density and catalyst strength). Silica-alumina catalysts such as in the present invention can be used “as is”, particularly in reactions that require acidic catalysts, or can optionally be combined with zeolites, clays or other binders, and inorganic oxides for the cracking of liquid hydrocarbons in cracking reactors such as fluid catalytic crackers.
Numerous silica-alumina catalyst compositions and processes for their preparation are described in the patent literature. The patent literature teaches a number of ways to prepare these compositions.
U.S. Pat. No. 4,499,197, Seese et al., for example, describes the preparation of inorganic oxide hydrogels, and more particularly, catalytically active amorphous silica-alumina and silica-alumina rare earth cogels. The active cogels are prepared by reacting aluminate and silicate solutions to obtain a silica-alumina pregel, and then reacting the pregel with an acidic rare earth and an aluminum salt solution with complete mixing. U.S. Pat. No. 4,239,651, Alfandi et al. discloses a process for preparing exchanged ammonium cogels.
U.S. Pat. No. 4,289,653, Jaffe teaches preparing an extruded catalyst by mixing aluminum sulfate and sulfuric acid with sodium silicate to form a silica sol in an alumina salt solution at pH of 1-3, adding NH4OH under substantially constant pH of at least 4 to 6; adding more NH4OH to form a cogelled mass to pH 7.5-8.5; washing cogelled mass; mulling the mass with peptizing agent, a Group VI-B metal compound and a Group VIII metal compound to form extrudable dough; extruding; and drying and calcining.
U.S. Pat. No. 4,988,659, Pecoraro teaches a cogelled, silica-alumina matrix prepared by the method which comprises mixing a silicate solution with an aqueous solution of an acid aluminum salt and an acid, to form an acidified silica sol in said aluminum salt solution, and adjusting said silica sol/aluminum salt solution mixture to a pH in the range of about 1 to 4; slowly adding sufficient base with vigorous stirring, to said acidified silica sol/aluminum salt solution mixture to form a cogel slurry of silica and alumina, and to adjust said slurry to a pH in the range of about 5 to 9; aging said cogel slurry at a temperature of ambient to 95° C.; adjusting the pH of said cogel slurry to about 5 to 9; recovering a cogelled mass from said slurry; washing said cogelled mass; adjusting the pH of said cogelled mass to between about 4 and 7, and controlling conditions to induce syneresis; and forming said combination into particles.